This is a very interesting promotion. The marketing talk states that if you do a stay that begins between Thursday and Saturday and book three nights, you will only pay for two – as long as you pay for them using your SPG American Express. Valid at most Aloft, Element and Four Points hotels. Promotion is here.
But the Terms & Conditions, accessible through a buried link, say that you don’t need to pay using an SPG card: you can pay using ANY American Express card. This is a nice bonus in case you do not have an SPG Amex (but if you do not have one, why not sign up? 🙂 ). Sadly, only US/Canada/Dominican Republic (??), but still, a good deal if you need it.